The Harrowing is the fiftieth short story published in the The Horus Heresy series that was not originally part of a main release anthology. The Harrowing was first published as part of the limited-release anthology Sedition's Gate, and was later made part of the War Without End anthology and as a stand-alone e-book.
The garrison world of Callistra Mundi is threatened by Traitor forces –- none other than the insidious Alpha Legion. Like so many before it, this attack will not come as a traditional Astartes assault, but as a Harrowing. Operatives and turncoats are revealed within the Mechanicum Ark freighter Omnissiax and her attendant battlegroup, and confusion will reign amongst the Loyalist defenders. And only when none can tell friend from foe will the true destruction begin...
- The Harrowing (Short Story) by Rob Sanders